A pitched battle or set piece battle is a battle in which both sides choose the fighting location and time, and where either side has the option to disengage either before the battle starts, or shortly after the first armed exchanges.
A pitched battle is not a chance encounter such as a skirmish, or where one side is forced to fight at a time not of their choosing such as happens in a siege. For example, the first pitched battle of the English Civil War, the Battle of Edgehill, was fought when the Royalists chose to move off an escarpment to a less advantageous position so that the Parliamentarians would be willing to fight.
Pitched battles may result from meeting engagement, where — instead of disengaging — the opposing generals choose to reinforce their positions and turn what was initially a skirmish into a pitched battle, as had happened in the Battle of Gettysburg, fought during the American Civil War.
Recreational battle reenactment tends to focus on pitched battles partially for sake of ease of demonstration.